In Leith (a port neighborhood of Edinburgh, Scotland) in the late 1980s, Rent Boy and Sick Boy begin experiencing heroin withdrawal symptoms. They go to see their dealer, Johnny Swan, a.k.a. “Mother Superior.” At Johnny’s place, Alison tells Rent Boy he should go see Kelly, who is feeling bad after having an abortion, but instead, Rent Boy just goes back and watches more Jean-Claude Van Damme.
Eventually, Rent Boy gets tired of managing his addiction and decides to try to go to a new apartment and quit cold turkey. The withdrawal symptoms get to be too much, however, and in his desperation to feel better, he ends up using opium suppositories, only to lose them in a fit of diarrhea and have to dig back through the toilet for them.
One day, while Rent Boy, Sick Boy, and several other characters are high, their friend Lesley comes in and announces that her baby, Dawn, has died, seemingly of sudden infant death syndrome. The event shakes everyone, particularly Sick Boy, who likely was Dawn’s father. Many of them promise to swear off heroin.
Second Prize and Tommy, two more of Rent Boy’s friends, witness a scene in a pub where a man violently assaults a woman. When they try to intervene, however, they’re shocked to hear the woman defending her abusive boyfriend. In a similarly violent pub scene, Rent Boy goes out with his “friend” Begbie (whom he actually despises) and their girlfriends, and Begbie impulsively throws a glass at a stranger’s head to start a fight. Begbie often picks fights with strangers and seems disappointed whenever they don’t fight back.
Although Rent Boy successfully kicks his heroin addiction for a while after Dawn’s death, eventually he relapses. Soon after, Tommy comes to visit him and asks for some heroin. Tommy has just broken up with his girlfriend, Lizzy. Although Rent Boy is reluctant at first, he eventually gives Tommy some heroin, sending him down the path to heavy addiction.
Rent Boy and his friends continue to do drugs and act recklessly. In one situation, a desperate Rent Boy decides to go home with one of the last remaining women at a nightclub. As it turns out, however, the “woman” (Dianne) is actually a girl of about 14 who is still in school. On another occasion, Rent Boy and his sweet-natured friend Spud rob a bookstore to resell the merchandise but get caught. Rent Boy manages to talk his way out of jail time, but Spud goes away to prison for six months.
Partly due to his drug use, Rent Boy has a strained relationship with his family, and so when he hears that his brother Billy dies while fighting for the British army in Ireland, Billy doesn’t feel any particular grief. He feels that his brother betrayed his Scottish identity by joining the British army, and partly as a result of these complicated feelings, he ends up having sex in a bathroom with his brother’s very pregnant former girlfriend. Ren Boy has a complicated relationship with sexuality. Later, when he meets an older gay man in London named Gi, he feels simultaneous disgust and sympathy, inspiring him to consider his own potential bisexuality.
Rent Boy later learns that one of his old friends Matty has died from complications of HIV, after getting an infection from the feces of a kitten Matty spontaneously adopted. Rent Boy comes back from London to Leith for Matty’s funeral and sees some of his old friends, many of whom he’s grown apart from. Later, Rent Boy returns to Leith again and visits Tommy, who has contracted HIV from sharing needles and who seems likely to die soon. Rent Boy also goes out walking with Begbie at an abandoned train station where they run into a drunken old man who jokingly asks them if they’re “trainspotting.” Rent Boy later realizes that the man was Begbie’s father. On their walk back from the train station, Begbie beats a man up, seemingly at random, and Rent Boy makes no move to stop him.
Several characters including Rent Boy, Sick Boy, Begbie, Spud, and Second Prize take a bus to London for one big heroin deal. Despite tensions within the group, the deal succeeds, and they make a lot of money. While they’re all out celebrating, however, Rent Boy walks off with the money for himself and books a flight to Amsterdam. While Rent Boy is nervous about starting a new life and has some regrets about hurting Spud by taking his share of the money, he is excited about getting a second chance in life.